1. Field of the Invention
This invention relates to underdrains in gravity filters and, more particularly, to an improvement for uniformly distributing backwash fluids into filter media using multi-lateral underdrains.
2. Description of the Prior Art
Various underdrain lateral designs for distributing backwash fluids, primarily water and, more recently, air/water mixtures, to filter media are well-known in the art. Three early dual lateral designs which proved successful are disclosed in U.S. Pat. No. 1,572,398 to Leopold, U.S. Pat. No. 2,378,239 to Myron and U.S. Pat. No. 3,110,667 to Stuppy, all of which are incorporated herein by reference. The general nature of these prior art designs is shown in FIGS. 1 and 2 of the drawings in the present application. Thus, FIGS. 1 and 2 show a filter block A having a plurality of walls which define two lower primary conduits B and two upper secondary conduits C. The primary and secondary conduits are separated by an interior wall D having a thickness T and also having metering orifices E therein for flow of backwash fluids from the primary conduits B into the secondary conduits C.
The blocks A are placed end-to-end in parallel, adjacent rows across a filter bottom and transverse to a flume to define underdrain laterals. The underdrain blocks A have been predominately made from clay or concrete. The thickness of the interior wall D in these designs is generally on the order of 5/8-1 inch.
FIGS. 3 and 4 of the drawings also show a dual lateral underdrain block 10 according to the prior art. Underdrain block 10 has a bottom wall 12, a top wall 14 with two sidewalls 16 extending therebetween. A vertical partition 18 is located within underdrain block 10 and extends between bottom wall 12 and top wall 14. A horizontal platform 20 extends between sidewalls 16. Partition 18 and platform 20 cooperate to define two lower primary horizontal conduits or "laterals" 22 and two upper secondary horizontal conduits 24. Horizontal platform 20 contains a plurality of metering orifices 26 which connect primary conduits 22 with secondary conduits 24. Top wall 14 contains a plurality of dispersion orifices 28 which connect the secondary conduits 24 with an overlying bed of granular media (not shown). Similar multi-lateral underdrain systems are shown in U.S. Pat. No. 4,065,391 to Farabaugh; U.S. Pat. No. 5,019,259 to Hambley; U.S. Pat. No. 5,068,034 to Walter; U.S. Pat. No. 4,691,765 to Roberts; and U.S. patent application Ser. No. 07/649,448, all of which are incorporated herein by reference. The invention described hereinafter may be applied to each of these multi-lateral underdrain systems.
According to Manifold Theory, as taught by Herbert E. Hudson, Jr. in the textbook Water Clarification Processes, Chapter 15, 1981, flow from a manifold, such as primary conduits B, into a perpendicular take-off conduit, such as orifices E, is impacted by the magnitude of the angle of repose of the manifold flow velocity as it enters the take-off conduits. The angle of repose is determined by the resolution of the vertical and horizontal components of the velocity. The uniformity of flow from manifold to take-off conduits generally improves as the angle of repose approaches 90 degrees with respect to the longitudinal axis of the manifold. The prior art clay and concrete underdrains have sufficient wall thickness and create enough frictional flow losses so that (a) the velocity within primary conduits B adjacent wall D is not unacceptably high, and (b) the angle of repose of flow velocity when entering orifices E does approach 90 degrees.
Today, it has been found desirable to mold underdrain blocks from plastic materials by foam or injection molding techniques. This generally provides for smaller wall thicknesses, on the order of 0.060-0.125 inches, and higher flow velocities at the interior wall D in which the metering orifices E are located. The ratio of the diameter of the metering orifices to the wall thickness can reach 2:1 or more, whereas the same ratio for prior art clay and tile underdrain blocks was approximately 1:1. The angle of repose of flow velocity as it enters the orifices E is generally less than 45 degrees. Maldistribution from the primary conduits B into the secondary conduits C under these circumstances is frequently encountered. The primary conduit metering orifice arrangement can no longer approximate a manifold and maldistribution can be as high as +/- 15% or more. Maldistribution of +/- 5% can be obtained in clay or concrete underdrains in some cases.
Attempts have been made to control fluid flow in modern day underdrain designs. For example, U.S. Pat. Nos. 417,405; 477,727; 1,668,205; 1,788,383; 4,118,322; 4,202,774; and 4,925,556 have addressed the problem either directly or indirectly.
Thus, it is an object of the present invention to improve backwash fluid distribution in today's thin-walled underdrain systems. It is a further object to meet and surpass the performance of prior art clay and concrete dual lateral underdrains with respect to maldistribution and headloss.